Jets of high-speed particles may be powered by magnetic fields and either the rotation of matter swirling around a black hole, the spin of the black hole itself, or both

(Image: NASA/CXC/M Weiss)

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The efficiency with which black holes convert the gas they eat into beams of high-speed particles spewed back into space has been measured for the first time. The observation bolsters the idea that the jets prevent new stars from forming in the black holes’ host galaxies, stunting their galactic growth.

Black holes millions or billions of times more massive than the Sun are thought to lie at the hearts of relatively large galaxies such as the Milky Way. Their intense gravitational pull draws in the gas around them, and a fraction of the energy from this consumption is released back into space.

Some of this energy takes the form of radiation and is produced when matter in a disc around the black hole is super-heated as it is being swallowed. Quasars, for example, are relatively short-lived “active” stages of black holes that are devouring their surroundings and are so bright they can be seen across the entire observable universe.

But the energy released can also take the form of matter that is accelerated to near-light speeds in jets that shoot out from the centre of the disc. Jets are thought to be powered by magnetic fields and the rotation of either the disc, the black hole itself, or both. (Watch an animation of the jets here (Mpeg&colon; NASA/CDC/A Hobart).

However, the role of the jets in the release of the black hole’s energy has not been clear. “The question is, are the jets a major player in the system, taking out a good bit of the total energy released?” asks John Hawley, an astronomer at the University of Virginia in Charlottesville, US. “How much stuff do you have to throw down a black hole to get jets out and how powerful would those jets be?”

Blasted cavities

Now, a team led by Steve Allen of Stanford University in California, US, has taken advantage of the high spatial resolution of the Chandra X-ray Observatory to answer those questions.

The team observed nine of the most massive, blob-shaped galaxies within 400 million light years of Earth. All were probably quasars at one time, but have since cut down on the amount of gas they eat and therefore do not shine brightly from their discs as quasars do.

The researchers measured the brightness and temperature of the central regions of the galaxies to determine the rate at which gas was falling into the black holes. Then, they measured the size of cavities blown out by the jets as they slammed into surrounding gas. This provided an estimate of the jets’ energy.

“For the first time, we measured both the fuelling rates of black holes and the amount of energy released in jets, allowing us to work out the efficiency of black hole engines,” Allen said at a press conference on Monday.

Boring and old?

The jets release about 2.5% of the energy the black holes consume. “That’s not too different from what I might have expected,” comments Hawley, who adds that quasars are thought to have efficiencies of about 10%.

But the research team members say the result is in fact surprising – these “dead quasars” are much more efficient than thought. Because the black holes studied are not bright like quasars, previous models had suggested they were messy, inefficient eaters and reject 95% of the “food” they tried to swallow, says Chris Reynolds of the University of Maryland in College Park, US.

They seemed to be “boring, old black holes we thought had stopped doing anything interesting a long time ago”, he says. “But our work shows that’s not true – they’re also efficient.” And based on the fact that all nine black holes share similar efficiencies, the jets may be stable for several million years – and possibly much longer, says the team.

The jets also have far-reaching effects, says Kim Weaver, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US. “They send out energy at enormous scales – hundreds of billions of times larger than the black holes themselves.”

The jets may heat the gas in their host galaxies, preventing it from cooling and clumping into new stars. “People are still not certain why the biggest galaxies in the universe aren’t even bigger,” she says. “This is one way to keep the stars from forming and the galaxies growing bigger.”

The research will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.